Liquid crystalline materials have come to be widely used as the active element in electronic devices which modulate the properties of light. The performance of these devices is in large part determined by the properties of the liquid crystal material. Nematic liquid crystals have been most widely utilized at this time, being used for the construction of several different forms of twisted-nematic type display devices such as are used for wrist watches, small computers and the like. Such displays are limited in response time by the nature of the nematic liquid crystalline phase and by the extent to which the structure of that phase must be perturbed to produce a useful optical response.
It has been demonstrated that liquid crystals possessing a ferroelectric phase can be utilized to fabricate useful electro-optic devices which exhibit response speeds far exceeding those attained to date with nematic liquid crystal materials (Clark and Lagerwall, Appl. Phys. Lett., 36, 899 (1980)). The present invention relates to a group of liquid crystalline materials which possess ferroelectric phases as pure materials, as mixtures with each other and as mixtures in smectic hosts. Such materials and mixtures are of use for electro-optical modulation in light shutters, flat panel displays and the like.
There are a host of critical material properties which must be fulfilled for application of such materials in a practical device. Among these critical requirements are the following:
1) Stable ferroelectric phase, particularly the S.sub.c * phase, at or near ambient temperature. Although an enantiotropic phase is preferred, materials with monotropic ferroelectric phases, and even monomers which of themselves have no ferroelectric phase are still of use in appropriate mixtures.
2) Large spontaneous polarization to provide efficient coupling of the external applied field to the ferroelectric liquid crystal contained in the device.
3) Fast electro-optical response to an applied electric field, especially in electronic display devices where a high degree of signal multiplexing exists.
It is the object of this invention to provide ferroelectric liquid crystal materials and compositions which satisfy these requirements.